In a chiller system, heat is removed from a secondary fluid flow. The secondary fluid flow may be a liquid flow, e.g. a flow of water or brine. As an alternative, the secondary fluid may be condensed during heat exchange. This is, e.g., the case when the chiller system is a pumped CO2 system, in which case the secondary fluid is CO2.
A chiller system comprises a primary side, often in the form of a vapour compression system, and a secondary side containing the secondary fluid flow. Heat exchange takes place between refrigerant flowing in the primary side and secondary fluid flowing in the secondary side, typically in an evaporator of the primary side.
The secondary fluid flow is typically driven by means of a pump. In order to ensure that a sufficient cooling capacity is available in situations of high cooling demand, the pump is normally operated at its highest level in all situations, also when it is not required by the cooling demand. Thereby the energy consumption of the pump is relatively high.
U.S. Pat. No. 6,871,509 discloses a chiller system with a primary side and a secondary side. The primary side is in the form of a vapour compression system comprising a compressor, a condenser, an expansion device and an evaporator. The secondary side provides a flow of water passing through the evaporator in such a manner that heat exchange takes place between the refrigerant of the primary side and the water of the secondary side. Thus, the evaporator absorbs heat from water circulating in the secondary loop. A pump is provided for driving the water flow through the evaporator. When the chiller system is activated, the pump begins circulating water through the evaporator. Thus, the pump is either operated at full speed, or is not operated at all.